The present invention relates to a magnetoresistive element or device using a magnetic layer having perpendicular magnetic anisotropy, and also to a magnetic memory using the magnetoresistive devices. The invention further relates to a recording method for such magnetic memories.
GMR (Giant MagnetoResistive) devices and TMR (Tunnel MagnetoResistive) devices obtained by stacking magnetic layers and nonmagnetic layers can be expected to exhibit higher performance as magnetic sensors, by virtue of their having larger rates of change in magnetoresistance as compared with conventional AMR (Anisotropic MagnetoResistive) devices. GMR devices have already been put into practical use as read-use or playback magnetic heads for HDDs (Hard Disk Drives). TMR devices, on the other hand, which have even higher rates of change of magnetoresistance than GMR devices, are under discussion for applications to not only magnetic heads but also magnetic memories.
A fundamental structure of a conventional TMR device as shown in FIG. 1 is known (see, for example, Japanese Patent Laid-Open Publication HEI 9-106514). Referring now to FIG. 1, the TMR device is made up by stacking a first magnetic layer 31, an insulating layer 32, a second magnetic layer 33 and an antiferromagnetic layer 34. In this case, the first magnetic layer 31 and the second magnetic layer 33 are ferromagnetics made of Fe, Co, Ni or alloys of these, the antiferromagnetic layer 34 is made of FeMn, NiMn or the like, and the insulating layer 32 is made of Al2O3.
Replacing the insulating layer 32 in FIG. 1 with a nonmagnetic layer having electrical conductivity made of Cu or the like would result in a GMR device.
In conventional GMR devices and TMR devices, since the magnetic layers are magnetized along the layer surfaces, scale-down of the device dimensions as in magnetic heads of narrow track widths or magnetic memories of high integration would cause those devices to be strongly affected by diamagnetic fields generated at end-portion magnetic poles. For this reason, the direction of magnetization of the magnetic layers would become unstable, which in turn makes it hard to maintain uniform magnetization, and eventually leads to occurrence of operating failures of the devices such as the magnetic head and the magnetic memory.
As a solution to this drawback, a magnetoresistive device using a magnetic layer having perpendicular magnetic anisotropy is disclosed in Japanese Patent Laid-Open Publication HEI 11-213650. The device structure as taught in this publication is shown in FIG. 2. The magnetoresistive device is so structured that a nonmagnetic layer 42 is sandwiched between a first magnetic layer 41 formed of a perpendicularly magnetized film having a low coercive force and a second magnetic layer 43 formed of a perpendicularly magnetized film having a high coercive force. The first and second magnetic layers are provided by using a ferrimagnetic film made of rare earthxe2x80x94transition element alloys, a garnet film, PtCo, PdCo or the like.
In this case, since end-portion magnetic poles occur at the magnetic film surfaces, increases in diamagnetic fields due to the scale-down of devices are suppressed. Accordingly, if perpendicular magnetic anisotropy energy of the magnetic film is substantially larger than the diamagnetic field energy caused by the end-portion magnetic poles, the magnetization can be stabilized along the perpendicular direction regardless of the device dimensions.
However, in the magnetoresistive device using a magnetic layer having perpendicular magnetic anisotropy, end-portion magnetic poles occur at the magnetic film surfaces. Since the nonmagnetic layer to be used for GMR devices and TMR devices is extremely thin, a magnetic pole that occurs at the interface of one magnetic layer and the nonmagnetic layer affects the magnetization of the other magnetic layer so largely that the magnetization may not be reversed. Accordingly, when the magnetoresistive devices are applied to a magnetic memory as an example, there may occur problems that information to be stored cannot be written to the memory or that written information is dissipated.
Therefore, with a view to solving these and other problems, a first object of the present invention is to provide a magnetoresistive device, as well as a magnetic memory using the magnetoresistive device, which allows a magnetic layer to be maintained in a stable magnetized state without being affected by a leakage magnetic field applied from the other magnetic layer through an insulating layer.
In the aforementioned conventional magnetoresistive device, in order that the magnetization within the magnetic layer overcomes the effect of the diamagnetic field energy due to the end-portion magnetic poles so as to be directed perpendicular stably, it is preferable that the perpendicular magnetic anisotropy energy of the magnetic film be as large as possible. However, this normally causes the coercive force to also increase concurrently. Accordingly, when a conventional magnetoresistive device having a sufficiently stabilized perpendicularly magnetized film is applied to a magnetic memory, the coercive force of the recording layer will be excessively increased, which would make it hard to perform magnetization reversal by a magnetic field generated by a recording current.
Therefore, a second object of the present invention is to provide a magnetoresistive device, as well as a magnetic memory using the magnetoresistive device, which has a coercive force of such a magnitude as to allow the magnetization reversal to be performed and which stably holds magnetized information stored in its recording layer.
Now, the recording method for a magnetic memory using a perpendicularly magnetized film is explained by way of an example disclosed in the Japanese Patent Laid-Open Publication HEI 11-213650. The arrangement of magnetoresistive devices and write lines according to the teaching of the publication is shown in FIG. 3.
The device of FIG. 3, as in the case of FIG. 2, is made up of a first magnetic layer 21, a nonmagnetic layer 22 and a second magnetic layer 23. Assuming that the first magnetic layer 21 is a memory layer, information recording to the device is fulfilled by passing electric currents through write or recording lines 24, 25 provided on both sides of the device to thereby make the magnetization of the first magnetic layer 21 reversed by the magnetic fields generated from the current lines. For example, to make the first magnetic layer 21 magnetized upward of the device, currents are passed through the recording line 24 frontward of the drawing sheet, namely toward a direction in which a front side of the drawing sheet is facing, and through the recording line 25 backward of the drawing sheet, namely toward a direction in which a reverse side of the drawing sheet is facing. Since the resultant of the magnetic fields 27 generated from these two current lines is directed upward of the device, magnetization of the first magnetic layer 21 can be directed upward of the device.
However, locating the recording lines beside the magnetoresistive device would be disadvantageous for high integration of the device. In the case where the recording lines are located on both sides of the device as shown in FIG. 3, with a wiring rule (F) used, the distance between the adjoining devices is 4F. On the other hand, in the case of an ordinary array pattern with no recording lines provided between devices, the distance between the adjoining devices is 2F. From the viewpoint that high integration of devices is of importance for memory fabrication, the arrangement of the magnetic memory of FIG. 3 is disadvantageous to the increase of the integration.
Further, in the wiring pattern shown in FIG. 3, although devices located beside a selected device are not recorded, devices located in the anteroposterior direction of the selected device (namely, in a direction perpendicular to the drawing sheet) would be recorded. As a consequence, it would be impossible to select a device at a point of intersection in a matrix shape in which the devices are arrayed.
Therefore, in view of these problems, a third object of the present invention is to provide a magnetic memory having a higher level of integration than the conventional counterpart, and a recording method therefor, which memory allows information to be recorded to devices or memory cells at points of intersection in a matrix-form arrangement of devices.
The present invention provides a magnetoresistive device comprising a first magnetic layer, a nonmagnetic layer, and a second magnetic layer, with the nonmagnetic layer being interposed between the first and second magnetic layers, the first and second magnetic layers having perpendicular magnetic anisotropy, characterized in that:
the magnetoresistive device has a structure such that magnetization information can be recorded thereto and that recorded magnetization information is stably maintained therein.
More specifically, in one embodiment, either the first magnetic layer or the second magnetic layer is made of a ferrimagnetic that has a compensation point around a room temperature.
With this arrangement, even if the size of the device is reduced, it is possible to reduce effects of end-portion magnetic poles, and thus reduce the disturbance due to leaked magnetic fields. As a result, the perpendicular magnetization state is stably held.
In one embodiment, the first magnetic layer is made of a ferromagnetic having a low coercive force as well as perpendicular magnetic anisotropy, while the second magnetic layer is made of a ferromagnetic having a high coercive force as well as perpendicular magnetic anisotropy, and the ferromagnetic forming the second magnetic layer is a rare earth metalxe2x80x94transition metal amorphous alloy that has a small saturation magnetization. In this embodiment, similar effects are obtained.
In one embodiment, the first magnetic layer is made of a ferromagnetic having a coercive force low enough to allow magnetization reversal of the first magnetic layer and a magnetic anisotropy energy high enough to maintain the perpendicular magnetic anisotropy of the first magnetic layer.
Using the magnetoresistive devices with this construction can provide a magnetic memory that is rewritable and that can stably hold recorded information.
In one embodiment, the magnetoresistive device further comprises a third magnetic layer having perpendicular magnetic anisotropy and a second nonmagnetic layer which is interposed between the second and third magnetic layers. In this case, the second magnetic layer is made of a ferromagnetic having a coercive force low enough to allow magnetization reversal of the second magnetic layer and a magnetic anisotropy energy high enough to maintain the perpendicular magnetic anisotropy of the second magnetic layer.
The present invention also provides a magnetic memory using, as memory cells, the magnetoresistive devices of any type as described above.
Also, the present invention provides a magnetic memory using magnetoresistive devices, each magnetoresistive device having a first magnetic layer, a nonmagnetic layer, and a second magnetic layer, with the nonmagnetic layer being interposed between the first and second magnetic layers, the first and second magnetic layers having perpendicular magnetic anisotropy, characterized in that:
current lines for recording information to the magnetoresistive devices are provided in planes upper and lower than a plane on which the magnetoresistive devices are located.
The current lines may be positioned directly above and below the magnetoresistive devices. Alternatively, the current lines may be positioned obliquely above and below the magnetoresistive devices such that they are located in positions between mutually adjacent magnetoresistive devices.
With the above arrangement, higher integration or packing density is achievable.
In one embodiment, the magnetoresistive devices are arranged in matrix shape, and the current lines in the upper plane extend in a direction crossing a direction in which the current lines in the lower plane extend.
In this magnetic memory, it is possible to record information selectively to a magnetoresistive device at a point of the matrix.
A layer of a high permeability material may be provided on each of the current lines. In this case, due to the presence of the high permeability layer, recording to the memory can be effectively performed.
In recording information to the magnetic memory as described above, electric currents are passed through two current lines that are positioned in the upper plane and adjacent to a selected magnetoresistive device while electric currents are passed through two current lines that are positioned in the lower plane and adjacent to the selected magnetoresistive device so that information is recorded to the selected magnetoresistive device.
Other objects, features and advantages of the present invention will be obvious from the following description.